1. Field of the Invention
Aspects of the present invention relate to a bipolar output voltage charge pump circuit, i.e. a single charge pump circuit which provides a pair of opposite polarity output voltages.
2. Description of the Related Art
Bipolar, i.e. dual rail, output voltage charge pump circuits are a type of DC-DC converter that utilize transfer and storage capacitors as devices to respectively transfer and store energy such that the converter is able to provide, from a unipolar, i.e. single rail, input voltage source, a bipolar output voltage that may be different in value from that of the unipolar input voltage.
In use, bipolar output voltage charge pump circuits may comprise output storage capacitors, typically known as “reservoir capacitors” and one or more energy transfer capacitors, typically known as “flying capacitors”. The terminals or connectors of the “reservoir capacitors” are permanently connected to respective output voltage terminals or nodes. In contrast, the terminals or connectors of the “flying capacitors” are capable of being switched, in a controlled sequence, to input or output voltage terminals or nodes or to the other flying capacitor terminals or nodes.
For example, a known bipolar output voltage charge pump circuit, as disclosed in the present applicants co-pending UK patent application GB 2444985, can provide positive and negative bipolar output voltages (+/−VDD/2) that are each equal to half the magnitude of the charge pump circuit's unipolar input voltage.
Furthermore, by suitable control, the co-pending UK patent application can also provide positive and negative bipolar output voltages (+/−VDD) that are each equal to the magnitude of the charge pump circuit's unipolar input voltage.
Such a known bipolar output voltage charge pump circuit uses an arrangement, i.e. a network, of switches, i.e. a switch matrix, to control the connection of the terminals of the two reservoir capacitors, i.e. the two output voltage terminals, and those of the flying capacitors. The flying capacitor terminals may be connected by these switches to: the input voltage terminal, i.e. the unipolar input voltage; the output voltage terminals, i.e. the bipolar output voltages; a reference terminal, e.g. ground potential; and to one another in order to obtain either the bipolar output voltage +/−VDD/2 or +/−VDD.
FIG. 1 schematically shows a known audio output chain 10, utilising a charge pump 12. The audio output chain 10 receives input audio signal data 14 and after processing (not shown) and amplifying the audio signal data by an amplifier 16, outputs an audio signal 18. Audio signal 18 may be output to a load 20, such as headphones, speakers or a line load, possibly via a connector (not illustrated) such as a mono or stereo jack.
As can be seen from FIG. 1, the charge pump circuit 12 receives an input supply voltage VV and a reference voltage VG, say ground, i.e. 0V, and is clocked by a clock signal CK. The charge pump circuit also has a flying capacitor 22. The output voltage VP, VN of the charge pump 12 may be +/−α.VV, where α may be 1 or 0.5. In this way, the output signal data 18, output from the amplifier 16, may be balanced around the reference VG, in this case ground.
Charge pump circuits, such as charge pump circuit 12 shown in FIG. 1, are widely used in portable electronics devices where decreasing power consumption in order to extend battery discharge time, is becoming ever more important. For an audio chain driving a 16 ohm headphone for example, typical listening levels in a quiet environment may require only 100 μW (40 mV rms or 2.5 mA rms for a 16 ohm headphone). However if this current is supplied from a +/−1.5V supply (required to drive 50 mW peaks for audibility in noisier environments) then the 2.5 mA rms sourced from the 1.5V supply consumes 3.3 mW, i.e. an efficiency of 100 μW/3.3 mW=3%.
Even if the supply voltage (VP, VN) can be halved using the known charge pump described above, then the efficiency is still poor, and reducing the power supply further makes it difficult practically to get enough voltage swing from the pre-drivers (not illustrated) to drive the output transistors of amplifier 16 properly.
Further, especially at low signal levels, the power required to switch the switching devices of the charge pump may be significant enough to degrade the efficiency.
Furthermore, in order to drive transducers such as piezoelectric transducers, haptic transducers or backlights for example, bipolar output voltages of greater than VV may be required. The same output chain may be required to drive such loads in some use cases, with a consequent requirement for operating modes with bipolar output stage supply voltages greater than VV.
It is desirable to be able to operate a particular charge pump circuit, particularly an integrated circuit implementation, in various applications which may have different supply voltages available. In order to maintain similar performance with different input supply voltages, it is desirable to have a range of step-down and step-up ratios available.
Charge pumps that generate a range of output voltages may have multiple flying capacitors. These flying capacitors are generally too large to be accommodated on-chip, so require dedicated pins on the package a well as occupying area on the PCB. It is desirable to minimise the number of flying capacitors to reduce cost, package size and board area.
It is therefore desirable to provide an audio output chain and an appropriate charge pump that can supply a wide range of output stage bipolar supply voltages to reduce or minimise power consumption over a wide range of output signal levels and input supplies, while providing a low cost and small physical size.